<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        /* body{
            background-color: black;
        } */
    </style>
</head>

<body>
    <button>开关</button>

    <script>
        const btnObj = document.querySelector("button")
        let flag = 0;
        btnObj.onclick = function () {
            if (flag == 0) {
                //把背景色变成黑色
                document.body.style.backgroundColor = "black"
                flag = 1
            } else {
                document.body.style.backgroundColor = "white"
                flag = 0;
            }

        }
    </script>
</body>

</html>